async function checkRegistration() {
        try {

            if (!window.fetch || !navigator.credentials || !navigator.credentials.create) {
                throw new Error('Browser not supported.');
            }

            // get check args
            let rep = await window.fetch('authServer.php?fn=getGetArgs' + getGetParams(), {method:'GET',cache:'no-cache'});
            const getArgs = await rep.json();

            // error handling
            if (getArgs.success === false) {
                throw new Error(getArgs.msg);
            }

            // replace binary base64 data with ArrayBuffer. a other way to do this
            // is the reviver function of JSON.parse()
            recursiveBase64StrToArrayBuffer(getArgs);

            // check credentials with hardware
            const cred = await navigator.credentials.get(getArgs);

            // create object for transmission to server
            const authenticatorAttestationResponse = {
                id: cred.rawId ? arrayBufferToBase64(cred.rawId) : null,
                clientDataJSON: cred.response.clientDataJSON  ? arrayBufferToBase64(cred.response.clientDataJSON) : null,
                authenticatorData: cred.response.authenticatorData ? arrayBufferToBase64(cred.response.authenticatorData) : null,
                signature: cred.response.signature ? arrayBufferToBase64(cred.response.signature) : null,
                userHandle: cred.response.userHandle ? arrayBufferToBase64(cred.response.userHandle) : null
            };

            // send to server
            rep = await window.fetch('authServer.php?fn=processGet' + getGetParams(), {
                method:'POST',
                body: JSON.stringify(authenticatorAttestationResponse),
                cache:'no-cache'
            });
            const authenticatorAttestationServerResponse = await rep.json();

            // check server response
            if (authenticatorAttestationServerResponse.success) {
                window.location.reload();
               /// window.alert(authenticatorAttestationServerResponse.msg || 'login success');

            } else {
                throw new Error(authenticatorAttestationServerResponse.msg);
            }

        } catch (err) {
            //reloadServerPreview();
            window.alert(err.message || 'unknown error occured');
        }
    }

    function queryFidoMetaDataService() {
            window.fetch('authServer.php?fn=queryFidoMetaDataService' + getGetParams(), {method:'GET',cache:'no-cache'}).then(function(response) {
                return response.json();

            }).then(function(json) {
               if (json.success) {
                   window.alert(json.msg);
               } else {
                   throw new Error(json.msg);
               }
            }).catch(function(err) {
                window.alert(err.message || 'unknown error occured');
            });
        }

        /**
         * convert RFC 1342-like base64 strings to array buffer
         * @param {mixed} obj
         * @returns {undefined}
         */
        function recursiveBase64StrToArrayBuffer(obj) {
            let prefix = '=?BINARY?B?';
            let suffix = '?=';
            if (typeof obj === 'object') {
                for (let key in obj) {
                    if (typeof obj[key] === 'string') {
                        let str = obj[key];
                        if (str.substring(0, prefix.length) === prefix && str.substring(str.length - suffix.length) === suffix) {
                            str = str.substring(prefix.length, str.length - suffix.length);

                            let binary_string = window.atob(str);
                            let len = binary_string.length;
                            let bytes = new Uint8Array(len);
                            for (let i = 0; i < len; i++)        {
                                bytes[i] = binary_string.charCodeAt(i);
                            }
                            obj[key] = bytes.buffer;
                        }
                    } else {
                        recursiveBase64StrToArrayBuffer(obj[key]);
                    }
                }
            }
        }

        /**
         * Convert a ArrayBuffer to Base64
         * @param {ArrayBuffer} buffer
         * @returns {String}
         */
        function arrayBufferToBase64(buffer) {
            let binary = '';
            let bytes = new Uint8Array(buffer);
            let len = bytes.byteLength;
            for (let i = 0; i < len; i++) {
                binary += String.fromCharCode( bytes[ i ] );
            }
            return window.btoa(binary);
        }

        /**
         * Get URL parameter
         * @returns {String}
         */
        function getGetParams() {
            let url = '';

            url += '&apple=1';
            url += '&yubico=1';
            url += '&solo=1';
            url += '&hypersecu=1';
            url += '&google=1';
            url += '&microsoft=1';
            url += '&mds=1';

            url += '&requireResidentKey=0';

            url += '&type_usb=1';
            url += '&type_nfc=1';
            url += '&type_ble=1';
            url += '&type_int=1';
            url += '&type_hybrid=1';

            url += '&fmt_android-key=1';
            url += '&fmt_android-safetynet=1';
            url += '&fmt_apple=1';
            url += '&fmt_fido-u2f=1';
            url += '&fmt_none=0' ;
            url += '&fmt_packed=1';
            url += '&fmt_tpm=1';
            url += '&userVerification=discouraged';

            return url;
        }